Exterior panels for motor vehicles are required to have low yield ratio properties in order to ensure shape fixability during forming processes. On the other hand, formed exterior panels in finished motor vehicles are required to have dent resistance so that they will not be easily deformed by external stress.
Bake-hardening steel is a kind of steel which can satisfy such both properties and in which solid solution carbon remains in the steel so that the yield strength of the final product can be increased by the diffusion of carbon to dislocations in a paint baking process to thereby ensure the dent resistance of the final product. Generally, bake-hardening steel guarantees an increase in yield strength of 3 kgf/mm2 or more.
However, solid solution carbon has some activity even under room temperature conditions other than paint baking conditions, and causes an aging phenomenon and yield point elongation.
The aging phenomenon occurs because solid solution carbon diffuses to mobile dislocations to interfere with the migration of the dislocations. The aging phenomenon also increases in proportion to the amount of solid solution carbon, and a method of controlling the amount of solid solution carbon in steel to about 0.001 wt % has been widely used to inhibit the aging phenomenon. However, the amount of solid solution carbon in steel is changed due to the components of the steel and various process variables in the steel production process, and the steel is exposed to conditions in which the aging phenomenon can occur at any time depending on the storage temperature of the steel.
It has been generally known that bake-hardening steels have aging resistance for 3 months at room temperature. However, in fact, the bake-hardening steels are required to have aging resistance for a longer period of time (about 6-12 months) when taking into consideration the transportation period and the time point of use.
Meanwhile, methods for improving the aging resistance of bake-hardening steel include a method of increasing the density of dislocations in the steel. However, in this method, if the amount of solid solution carbon is large or if low reduction ratio is applied or if suitable temper rolling cannot be performed due to operating conditions, some yield point elongation will remain to cause surface defects, or aging will proceed rapidly within a short period of time to reduce the quality of the steel.
Prior art documents related to the present invention include Korean Patent Laid-Open Publication No. 10-2000-0016460 (published on Mar. 25, 2000), entitled “Coated seizure hardening type cold-rolled steel sheet and production method thereof”.